About half of all grinding wheels made are vitrified bonded, and most of the rest are phenolic resin bonded. Wheels formulated with a liquid epoxy resin bond are the latest development. Liquid epoxy resin has superior physical characteristics as compared to phenolic resin. Among other aspects, it has higher tensile strength, less brittleness, more heat resistance, extra adhesion and better resistance to coolants. Powdered phenolic resin, however, is much easier to use in the production of wheels, and so is better suited for making a wide variety of wheel formulations.
The term "grinding wheels" refers to hard grinding elements including the standard wheel shape, cup wheel shape, mounted point shape, honing stone shape, etc. Grinding wheels are made of abrasive grains held together in a matrix by a binding material or ingredient.
The main binding ingredient is usually classified as being inorganic or organic. Inorganic binders include ceramic (vitrified) and oxychloride (magnesite). Organic binders include resins such as phenolic, shellac, epoxy, polyester and rubber. In some cases, the binder may be a combination, such as a phenolic resin and rubber. Different applications may require different binding materials.
In addition to the main binder material, other materials or fillers are often incorporated in a wheel formulation. These may be for the purpose of increased wheel strength, lubrication at the point of grind, prevention of steel chips welding to wheel face, or other such processing and operating benefits.
The major operations in the manufacturing of grinding wheels are: mixing the ingredients, molding them, firing them, and finishing them. Mixing is the operation of weighing raw materials, combining them in a mixing machine, screening them, and related steps prior to molding. Molding is the operation of placing the mix of raw materials in a steel form, and then leveling and pressing the mix to the desired thickness to form a "green" wheel which may be handled without falling apart. Firing or curing is the operation of applying heat in a kiln or oven to the molded "green" wheel in order to fuse an inorganic bond or polymerize an organic bond and producing a grinding wheel which is then cooled. Finishing is the operation of trimming the hardened wheel so as to remove material in excess of the desired final size.
The combined raw material mix for the grinding wheel generally can be classified as being dry, liquid (fluid or viscous), or a resilient solid. Most grinding wheels, whether inorganic or organic bonded, are produced from a dry mix. The abrasive grain is wetted thinly with a wetting agent--a solvent and/or resinous liquid--and then blended with a powdered binder. The binder is slightly dissolved by the wetting agent and adheres to each abrasive grain as a coating.
Inorganic or organic bonded wheels of a more specialized nature are produced from a liquid mix. The abrasive grain is coated with a mainly liquid binder.
Rubber bonded wheels are produced from slabs of resilient rubber impregnated with abrasive grain and fillers. This type of grinding wheel with a rubber mix is a minor one and has become more so in recent years.
The dry mix process of making grinding wheels generally is regarded as the most efficient and that is the reason for its predominance in the industry. It offers production and consistency advantages unmatched by the less used alternative liquid mix process.
The main production advantage of the dry mix process lies in its suitability for automation of mixing and molding. As a consequence, the labor content of the dry mix process is usually significantly lower than that with the liquid method. Large batches of powdered mix may be blended with ease in any of a number of common type industrial mixers. Because all ingredients, except for a small amount of wetting agent, are dry, the handling of them before and after mixing offers little difficulty. The equipment, including material pans, mixer blades, mixer chamber, chutes and the like, remains dry and relatively clean.
The liquid method, in most cases, does not lend itself to the same level of mixing automation, and requires more labor and complex equipment. Liquid epoxies, urethanes and some phenolics generally require the addition of curing agents, accelerators and the like. Precise proportions must be maintained. Most curing agents and accelerators are liquid, and they start resin advancement when added. Because of the viscosity, proportion sensitivity, and resin advancement, volume wheel production requires sophisticated component metering and mixing equipment. Even such equipment, however, cannot avoid many problems associated with mixing liquids with large quantities of sand-like abrasive grains. Accumulation of viscous materials, especially room temperature curing types, on equipment is a serious problem, and often requires removal by application of hazardous solvents. Wheels can be made by the liquid method by using less automated equipment but at a cost of extra labor and product inconsistency.
The dry molding operation offers labor savings over liquid methods. The dry method of molding is widely used and consists of pouring the free-flowing dry mix into a steel circular mold rotating on its vertical axis. After the proper amount of dry mix has been poured into the mold and leveled, the mold rotation is halted. A steel plate, similar to one already below the mix, is placed on top of the mix. The mold then is transferred into a hydraulic press, and the press is closed onto the mold assembly and exerts great compressive force on the wheel mix. When the mix has been compressed to the desired thickness it forms a "green" wheel, and the press ram retracts and the mold is moved onto a stripping mechanism. The stripper clamps the mold while pushing or stripping the "green" wheel upward from the mold. The mold then is moved back to its original location for the next cycle. This sequence of molding steps often is semi- or fully automatic and requires only a few moments to complete, depending on wheel size. The "green" wheel, after pressing, possesses sufficient "green" or uncured strength to withstand stripping and handling forces. It then is placed on a suitable plate or cart and transported into an oven.
Liquid molding does not have the advantage of using one mold for a rapid succession of wheels. Because the mix is wet or damp, once placed in a mold, it must stay until partly or fully cured. It has no "green" strength. Demolding may be delayed from several hours to over a day, depending on whether heat is applied. If heat is applied, extra time may be needed to cool the mold before cycling again. A number of molds are required even for modest production levels. Although a hydraulic press may not be necessary, other methods such as tamping or rolling are often used to fill the mold properly. Liquid mix must be poured into the mold carefully since trapped air bubbles can affect the quality of the finished wheel. Consistency of finished product is often a problem. If using a reactive mix, the last of the batch will have advanced somewhat by the time it is molded into a wheel. As a consequence, the last wheel of the batch may grind differently from the first.
An important feature of most grinding wheels is the porosity of the structure. Basically, a wheel is comprised of three entities: abrasive grains, a bond coating each grain and attaching it to its neighbor to form a matrix, and voids that exist between the grains of the matrix. The voids perform a useful function in the manufacturing and performance of a properly designed wheel. After curing, their similar size and frequency on all sides of the wheel indicates structure uniformity. Liquid bonded wheels can have the difficulty of the grains sinking to the bottom of the mold, leaving an excess of bonding material on top. The structure would not be considered uniform. During grinding, the voids provide space for metal chips from the object being ground to lodge temporarily. The larger the chips, the larger should be the designed voids. They also provide space for coolant to occupy, and allow the coolant to better reach the area of grind. The dry process naturally produces a porous structure. The liquid process does not, and must include special fillers that, upon burning away, leave voids in their place. The effectiveness of such fillers is often questionable.
A problem with all phenolic formulations, and most liquid epoxy formulations, is the presence of environmentally undesirable compounds. Phenolic resin bonds are based on the simultaneous use of phenolic resoles and phenolic novolacs. During a two-day curing operation at 175 degrees C., significant quantities of free phenol, formaldehyde and ammonia are released into the air. Epoxy resins generally are not an environmental problem but the curing agents can be hazardous. The most commonly used curing agents for liquid epoxies are aliphatic polyamines. These are classified as skin sensitizers, and can cause respiratory difficulties. Aromatic amines also are used for this type product and they are classified similarly as well as being a suspected carcinogen. Reactive diluents used for reducing liquid resin viscosity are sensitizing agents, and must be handled with care. Because of the nature of grinding wheel manufacturing, close physical and respiratory contact with materials in the process is nearly impossible to avoid.
Simply put, the dry process is the most effective method of designing and manufacturing grinding wheels. The resin with the best physical properties, however, is epoxy, which is a liquid in its commonly used form. Both phenolic and liquid epoxy bonds can be hazardous to workers and the environment.